Post rebuild engine failure

[eprovenzano]

I rebuilt my son's 400ex over the winter months. I replaced the original 10 year old piston and rings with a new Wiseco piston and ring (STD bore), and had the valve seals rebuilt and the cylinder honed. I did have some issues, which I was able to work through it, mostly getting the valves adjusted properly.

My son rode the quad a few times, and he reported that it really ran well. It had a lot more power and did not use a drop of oil....

Until the last ride... it died. Of course it was over 2 miles deep in the woods, and of course it was all up hill back to the truck.

OK, checked the valves, all was ok... cam looked fine, timing was dead on. Pulled the valve head... and i see shinny bits on top of the piston. Move the piston to the bottom of the stroke... ouch, the cylinder is scarred... Pull the cylinder and the rear portion of the piston (carb side) looks to have had a melt down. The rings are stuck and the cylinder will need to be bored to clean it up.

OK, what would have caused this? My son said he thought it got hot, but I have nothing to go on at this point. My 1st thought was it has to be lean, but the plug was black, as was the combustion chamber showing if anything it was rich. I'm stumped, I will rebuild, but just not sure what would have caused this...

Oh and I will not be using Wiseco piston next time, I think I’ll go with a Pro-X to eliminate the piston increasing in size when warm/hot.

[honda400ex2003]

my guess would be improper jetting. what size piston is in it now? any mods done to it? was he riding quite hard when it died? what was the jetting? i for one would much rather have a wiseco than a pro x. steve

[eprovenzano]

I don't have the jetting written down, but it was fine prior to the rebuild. It was still on the OEM piston and rings (10 years). The only thing I adjusted post rebuild was the air screw adjustment. I think turned it in so (I'm going from memory) it was only out one and one quarter turns.

I peronally did the break-in so I know it was broken in properly. He rode it two times and then it failed. Was he riding it hard, no not really. The 1st ride was with a friend who is very slow. He spent most of the ride helping his buddy out of jams. The 2nd ride, was another one of his buddies, much better rider, but the 1st time on out trails, so he was not pushing it tooo hard.

I plan to get the jetting written down, just haven't had time to do it.

[honda400ex2003]

ok so it was a stock set of rings and piston that was put in new. did you do a hone before putting the new stuff in the cylinder or just bolt them in? you leaned it out more by turning the f/a in also. the stock stuff would be a 148 main, 3rdd clip position on the needle, 38 pilot and 2.25 turns out on the f/a screw. steve

[JOHNDOE83]

Maybe the oil pump took a dive?

[togup]

sound more like improper bore to me , possibly an egg shape bore or the new piston was to large for the jug even though it was a standard bore

[togup]

hey it sucks to blow a new engine man I know where your coming from, I suggest boring it to 416 or 426 ,a new piston something like 10.5-1 and see what these guys on here are using for jetting

[eprovenzano]

Yes I did have the cylinder honed. I'm guessing on the a/s setting, but I do know I did turn in more than stock. In the back of my mind, I felt it was running lean... but I just couldn't prove it. I guess I was right...

Unfortuntly, I'm working out of town for the next month or two.... so I'm only going to be able to work on the bike during the weekends..

[JOHNDOE83]

I doubt its a jetting issue, unless it was backfiring alot after the install.

It has to be a flaw in the install, or oil related.

My 440 ran for three month's with stock jetting, never blew up, but did pop and surge alot and got really hot....Then I jetted it.

Was there alot of backfiring? if not I dont think it was jetting.

Based on it being a std bore piston.

[eprovenzano]

It was backfiring through the carb, but I did get it sorted out. The a/s adjustment smoothed that out.

I blame my son.... he wanted me to bore it out to a 416 during the 1st rebuild.... so now I think he may get his wish.... :D

There had to be something not quite "right". To me something was off, and I just can't put my finger on it. In the back of my mind I felt it was running lean, but I couldn't prove it. I'm still not sure what the cause was, so I've enlisted the help of a friend who recently completed a case splitting rebuild of his 400ex.

I'm betting it was something I did incorrectly. Either way, I'll get the cylinder off to the machine shop get it prepped to be bored out.

[gojk]

I sounds like a lean condition except for the fact that you said it melted on the intake side. Usually when lean...It will melt on the exhaust side. It still could have been lean though, did you check for a leak in the intake or exhaust? I doubt an exhaust leak would cause that lean of a condition though but an intake leak could have easily caused the problem.

There isn't much reason to debate what happened now though. Bore it out and throw a new piston in it. Check everything when you put it back together and you will make sure it doesn't happen again. Good luck!

[jcs003]

where the rings properly installed?

was the piston you installed a higher compression piston? if so, having the wrong octane fuel will eventually cause failure.

maybe it wasnt broke in properly. wiesco pistons need alot of break in time. and if done wrong they never run correct.

another thing. after 10 years the bore and piston could of wore into each other in an oval or egg shape. you should of checked it for being round within factory tolorences.

[eprovenzano]

I didn't check the cylinder, but I did send it to a machine shop to be honed. I don't know if the cylinder was checked or not, I "assumed" the machine shop would have checked it. I will be using a different machinists this time around. :D

[eprovenzano]

I finally had some time to work on the quad. The wristpin was fused to the piston. I had to use a saw to cut relief’s and then use a big *** C clamp to get the pin out.

I still have the original piston, and was looking the cylinder. The piston is definitely undersized.... mmmm There is no doubt not what happened. The bore was too large and my machinist did not check it. I did not ask, nor did I check it. I "***"umbed it was fine. NOT...

I purchased a used cylinder from an 08, it will not be here until next week. You can bet your last dollar, I will have that cylinder checked prior to putting it back together.

One last thing, I "ASSume a cylinder from an 08 will work on my 99... Can someone please verify that for me.

Thanks.